This invention relates to variable timing tappets for use in internal combustion engines. More specifically, it relates to pressure limiting tappets to vary the effective profile of a camshaft by hydraulically extending the drive train between the camshaft and a camshaft operated mechanism. Although hydraulic tappets are known in the prior art, they are not pressure sensitive, they rapidly collapse when the high pressure hydraulic fluid is vented, and they vent the fluid onto the engine block rather than confining it to the fluid supply system.
In an effort to maximize the efficiency and power output of diesel engines, as well as lower undesirable exhaust emissions, diesel engine manufacturers have sought a reliable and consistent means of altering the timing of injection and the opening and closing of cylinder valves. In a typical diesel engine, the injectors and valves are operated by a camshaft having a plurality of precisely defined lobe profiles radially located in a timed rotational relationship. Each lobe is connected to a camshaft operated mechanism, such as a valve or injector, by a suitable combination of mechanical links, including push rods, rocker arms, etc. However, such mechanical linkage results in a rigid timing program that cannot be altered while the engine is operating.
In the past, manufacturers have experimented with a variety of means to alter engine timing, but most have not proven successful. These efforts have included eccentric cam followers, gear phasers, overtravel tappets, helix combination injectors, hydraulic intensifiers, and variable task hydraulic tappets.
Prior art variable length hydraulic tappets have also been used to vary timing, but they have met only limited success. Hydraulic tappets are interposed between the camshaft and camshaft operated item and alter engine timing by selectively lengthening the timing drive train, thereby changing the effective profile of a camshaft. Typically, the collapsed or shortened tappet permits the camshaft operated item to operate in its normal timing sequence. When the tappet is lengthened, by trapping hydraulic (non-compressible) fluid in an internal tappet chamber, the drive train between the camshaft and camshaft operated item is lengthened, advancing the normal timing sequence. Conversely, such a tappet may be used to retard timing by selectively collapsing it to shorten the camshaft drive train.
However, these tappets suffer various deficiencies, including sensitivity to hydraulic fluid viscosity and engine speed, manufacturing tolerances, non-uniform pressure maintenance, failure to self prime from dry engine start up, irregular transient response, an excessive failure rate due to the high hydraulic pressures generated, and energy losses in the continued pumping of the hydraulic fluid. In addition, variable length hydraulic tappets are limited to use only where the camshaft operated item is not sensitive to increased pressure loading, cam link overtravel, and rapid tappet collapse. Thus, such prior art hydraulic tappets are unsuitable for use with highly loaded unit injectors and are restricted to use with camshaft operated valves for a variety of reasons. Specifically, camshaft link overtravel and increased injection camshaft pressure or train loads may burst the injector cup, reduce injection duration and throttle fueling in an advance mode. Also, rapid tappet collapse may interfere with a sharp, clean end of injection and permit hot exhaust gases to escape into the injector.